1. Field of Invention
This invention relates to a process for the preparation of monomeric methylolmelamines which are partially etherified with methanol or ethanol and which contain at least 4 moles of formaldehyde per mole of melamine, in a one-stage reaction, without base catalysis for the methylolation reaction and without acid catalysis for the etherification reaction.
Compounds of this type are already known and have, for example, the following formula: ##STR1## there being, on average, 4 to 5.5 moles of formaldehyde to 1 mole of melamine, and 1 to 4 methylol groups being etherified. This means that both the degree of methylolation and the degree of etherification of the compounds, which are obtained in the form of a mixture, may be different.
Compounds or mixtures of this type are employed, for example, in the manufacture of paints or textiles.
2. Description of the Prior Art
Numerous processes for the preparation of these compounds are already known; in these processes the methylolation is carried out under alkaline conditions; above all in order to achieve high degrees of methylolation, but the etherification is carried out at acid pH values. Examples of such processes are disclosed in U.S. Pat. Nos. 2,529,856 2,715,619 and 2,918,452, British Pat. Nos. 1,030,268 and 1,579,416, U.S. Pat. No. 4,223,141. Also European Patent Application No. 17,887, discloses a process in which strong acids must be present in the etherification stage and are preferred to fairly weak acids.
All the above prior art processes are carried out, at least in the methylolation stage, with the addition of water, which is in most cases introduced into the reaction mixture in the form of an aqueous formaldehyde solution or an aqueous alkali metal hydroxide solution.
European Patent Application No. 17,887 also discloses, inter alia, carrying out the reaction without the addition of water, which is recommended in cases where low degrees of condensation are desired. The etherified methylolmelamines thus obtained have the disadvantage that they have a considerable content of salt and also, as a result of hydrolysis, frequently contain acid constituents which impair their stability on storage because further condensation reaction to give resins is thereby promoted. This inadequate durability can be counteracted by rendering the mixtures alkaline for storage, which in turn increases the salt content even further and which can cause problems when processing, for example, in lacquers.
In addition to these two-stage processes, a one-stage process for the manufacture of etherified methylolmelamines is disclosed in German Auslegeschrift No. 2,516,349, in accordance with which melamine, formaldehyde and the alcohol used in the etherification stage are reacted in the presence of an acid at a pH value between 3 and 6.5, under elevated pressure and at a temperature within the range of 80.degree. to 130.degree. C. Besides the short reaction time, the advantage of this process is stated to be the production of products having fairly low degrees of methylolation. However, the disadvantage of the process is that even after quite short reaction times, high degrees of condensation occur, which cause the resins to have a limited solubility in water or to be completely insoluble in water. The tendency to crosslinking may be restrained only by extremely short reaction times of, for example, only 1 minute. The recommendation to carry out the reaction under anhydrous conditions is evidently not successful, as can be seen from the Examples. To avoid the stated high degrees of crosslinking, the same Applicant therefore has reverted to the two-stage procedure using alkaline methylolation, as is disclosed in European Patent Application No. 17,887.